setns(2)

Synopsis

Description

Given a file descriptor referring to a namespace,
reassociate the calling thread with that namespace.

The
fd
argument is a file descriptor referring to one of the namespace entries in a
/proc/[pid]/ns/
directory; see
namespaces(7)
for further information on
/proc/[pid]/ns/.
The calling thread will be reassociated with the corresponding namespace,
subject to any constraints imposed by the
nstype
argument.

The
nstype
argument specifies which type of namespace
the calling thread may be reassociated with.
This argument can have one of the following values:

0

Allow any type of namespace to be joined.

CLONE_NEWIPC (since Linux 3.0)

fd
must refer to an IPC namespace.

CLONE_NEWNET (since Linux 3.0)

fd
must refer to a network namespace.

CLONE_NEWNS (since Linux 3.8)

fd
must refer to a mount namespace.

CLONE_NEWPID (since Linux 3.8)

fd
must refer to a descendant PID namespace.

CLONE_NEWUSER (since Linux 3.8)

fd
must refer to a user namespace.

CLONE_NEWUTS (since Linux 3.0)

fd
must refer to a UTS namespace.

Specifying
nstype
as 0 suffices if the caller knows (or does not care)
what type of namespace is referred to by
fd.
Specifying a nonzero value for
nstype
is useful if the caller does not know what type of namespace is referred to by
fd
and wants to ensure that the namespace is of a particular type.
(The caller might not know the type of the namespace referred to by
fd
if the file descriptor was opened by another process and, for example,
passed to the caller via a UNIX domain socket.)

CLONE_NEWPID
behaves somewhat differently from the other
nstype
values:
reassociating the calling thread with a PID namespace changes only
the PID namespace that child processes of the caller will be created in;
it does not change the PID namespace of the caller itself.
Reassociating with a PID namespace is allowed only if the
PID namespace specified by
fd
is a descendant (child, grandchild, etc.)
of the PID namespace of the caller.
For further details on PID namespaces, see
pid_namespaces(7).

A process reassociating itself with a user namespace must have the
CAP_SYS_ADMIN
capability in the target user namespace.
Upon successfully joining a user namespace,
a process is granted all capabilities in that namespace,
regardless of its user and group IDs.
A multithreaded process may not change user namespace with
setns().
It is not permitted to use
setns()
to reenter the caller's current user namespace.
This prevents a caller that has dropped capabilities from regaining
those capabilities via a call to
setns().
For security reasons,
a process can't join a new user namespace if it is sharing
filesystem-related attributes
(the attributes whose sharing is controlled by the
clone(2)
CLONE_FS
flag) with another process.
For further details on user namespaces, see
user_namespaces(7).

A process may not be reassociated with a new mount namespace if it is
multithreaded.
Changing the mount namespace requires that the caller possess both
CAP_SYS_CHROOT
and
CAP_SYS_ADMIN
capabilities in its own user namespace and
CAP_SYS_ADMIN
in the target mount namespace.
See
user_namespaces(7)
for details on the interaction of user namespaces and mount namespaces.

Return Value

On success,
setns()
returns 0.
On failure, -1 is returned and
errno
is set to indicate the error.

Errors

EBADF

fd
is not a valid file descriptor.

EINVAL

fd
refers to a namespace whose type does not match that specified in
nstype.

EINVAL

There is problem with reassociating
the thread with the specified namespace.

The caller attempted to join the user namespace
in which it is already a member.

EINVAL

The caller shares filesystem
(CLONE_FS)
state (in particular, the root directory)
with other processes and tried to join a new user namespace.

EINVAL

The caller is multithreaded and tried to join a new user namespace.

ENOMEM

Cannot allocate sufficient memory to change the specified namespace.

EPERM

The calling thread did not have the required capability
for this operation.

Versions

The
setns()
system call first appeared in Linux in kernel 3.0;
library support was added to glibc in version 2.14.

Conforming To

The
setns()
system call is Linux-specific.

Notes

Not all of the attributes that can be shared when
a new thread is created using
clone(2)
can be changed using
setns().

Example

The program below takes two or more arguments.
The first argument specifies the pathname of a namespace file in an existing
/proc/[pid]/ns/
directory.
The remaining arguments specify a command and its arguments.
The program opens the namespace file, joins that namespace using
setns(),
and executes the specified command inside that namespace.

The following shell session demonstrates the use of this program
(compiled as a binary named
ns_exec)
in conjunction with the
CLONE_NEWUTS
example program in the
clone(2)
man page (complied as a binary named
newuts).

We begin by executing the example program in
clone(2)
in the background.
That program creates a child in a separate UTS namespace.
The child changes the hostname in its namespace,
and then both processes display the hostnames in their UTS namespaces,
so that we can see that they are different.

See Also

Colophon

This page is part of release 3.80 of the Linux
man-pages
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and the latest version of this page,
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